For optical fibers, the refractive index profile is generally set forth in terms of the difference in value between two points on the graph of the function associating the refractive index with the radius of the fiber. Conventionally, the distance r to the center of the fiber is shown along the x-axis of the profile. The difference between the refractive index at distance r and the refractive index of the external fiber cladding is shown along the y-axis (FIG. 2, references 21-24). The external cladding functions as an optical cladding and has a substantially constant refractive index. This optical cladding is generally composed of pure silica but can also contain one or more dopants. The optical fiber refractive index profile is referred to as a “step” profile, a “trapezoidal” profile, or a “triangular” profile for graphs having the respective shapes of a step, a trapezoid, or a triangle. These curves are generally representative of the theoretical or reference index profile (i.e., set profile) of the fiber. Fiber manufacturing constraints may lead to a slightly different profile in the actual fiber.
An optical fiber is conventionally composed of (i) an optical core, having the function of transmitting and optionally amplifying an optical signal, and (ii) an optical cladding, having the function of confining the optical signal in the core. For this purpose, the refractive indexes of the core (nc) and of the cladding (ng) are such that nc>ng. As is well known in the art, the propagation of an optical signal in a single-mode optical fiber is broken down into a fundamental mode (known as LP01) guided in the core, and into secondary modes guided over a certain radius in the core-cladding assembly.
Conventionally, step-index fibers, also called SMF fibers (“Single Mode Fibers”) are used as line fibers for optical fiber transmission systems. These fibers exhibit a chromatic dispersion and a chromatic dispersion slope corresponding to specific telecommunication standards.
For the requirements of compatibility between the optical systems from different manufacturers, the International Telecommunication Union (ITU) has defined a recommended standard with a norm, referenced ITU-T G.652, which must be met by a Standard Single Mode Fiber (SSMF).
This G.652 standard for transmission fibers recommends inter alia, a nominal range of 8.6 microns (μm) to 9.5 microns (μm) for the Mode Field Diameter (MFD) at a wavelength of 1310 nanometers, which can vary with +/−0.4 micron (μm) due to manufacturing tolerances; a maximum of 1260 nanometers for the cable cut-off wavelength; a range of 1300 nanometers to 1324 nanometers for the dispersion cancellation wavelength (denoted λ0); and a maximum chromatic dispersion slope of 0.092 ps/(nm2·km) (i.e., ps/nm2/km).
The cable cut-off wavelength is conventionally measured as the wavelength at which the optical signal is no longer single mode after propagation over 22 meters of fiber, such as defined by Subcommittee 86A of the International Electrotechnical Commission in the IEC 60793-1-44 standard. In most cases, the secondary mode most resistant to bending losses is the LP11 mode. The cable cut-off wavelength is, therefore, the wavelength beyond which the LP11 mode is sufficiently weakened after propagation over 22 meters of fiber. The method proposed by the standard involves considering that the optical signal is single mode when the attenuation of the LP11 mode is greater than or equal to 19.3 dB.
Moreover, for a given optical fiber, a so-called MAC value is defined as the ratio of the mode field diameter of the fiber at 1550 nanometers over the effective cut-off wavelength λceff. The cut-off wavelength is conventionally measured as the wavelength at which the optical signal is no longer single mode after propagation over two meters of fiber, as defined by Subcommittee 86A of the International Electrotechnical Commission in the IEC 60793-1-44 standard. The MAC constitutes a parameter for assessing the performances of the fiber, in particular for finding a compromise between the mode field diameter, the effective cut-off wavelength, and the bending losses.
Commonly assigned U.S. Patent Application Publication No. US2007/0280615, now U.S. Pat. No. 7,587,111, (and its counterpart European Patent Application No. 1,845,399) and commonly assigned U.S. Patent Application Publication No. US2007/0127878, now U.S. Pat. No. 7,623,747, (and its counterpart European Patent Application No. 1,785,754) disclose a relationship between the value of the MAC at a wavelength of 1550 nanometers and the bending losses at a wavelength of 1625 nanometers with a radius of curvature of 15 millimeters in a standard step-index fiber (SSMF). Each of these published patent applications is hereby incorporated by reference in its entirety.
Furthermore, each application establishes that the MAC influences the bending losses of the fiber and that reducing the MAC value reduces these bending losses. Reducing the mode field diameter and/or increasing the effective cut-off wavelength reduces the MAC value but may lead to noncompliance with the G.652 standard, making the fiber commercially incompatible with some transmission systems.
The reduction of the bending losses, while retaining certain optical transmission parameters, constitutes a challenge for Fiber-To-The-Home (FTTH) applications.
The International Telecommunications Union (ITU) has also defined recommended standards referenced ITU-T G.657A and ITU-T G.657B, which must be met by the optical fibers intended for FTTH applications, particularly in terms of resistance to bending losses. The G.657A standard imposes limits on values for bending losses but seeks, above all, to preserve compatibility with the G.652 standard, particularly in terms of mode field diameter (MFD) and chromatic dispersion. On the other hand, the G.657B standard imposes strict bending loss limits, particularly (i) bending losses less than 0.003 dB/turn at a wavelength of 1550 nanometers for a radius of curvature of 15 millimeters, and (ii) bending losses less than 0.01 dB/turn at a wavelength of 1625 nanometers for a radius of curvature of 15 millimeters.
Commonly assigned U.S. Patent Application Publication No. US2007/0280615, now U.S. Pat. No. 7,587,111, (and its counterpart European Patent Application No. 1,845,399) and U.S. Patent Application Publication No. US2007/0127878, now U.S. Pat. No. 7,623,747, (and its counterpart European Patent Application No. 1,785,754) propose fiber profiles having limited bending losses, corresponding in particular to the criteria of the G.657A and G.657B standards. The profiles described in these European patent applications, however, make it possible to achieve only the bending loss limits imposed by the G.657B standard.
U.S. Pat. No. 7,164,835 and U.S. Pat. No. 7,440,663, each of which is hereby incorporated by reference in its entirety, also describe fiber profiles exhibiting limited bending losses. The disclosed fibers, however, correspond only to the criteria of the G.657A and G.657B standards, particularly in terms of mode field diameter and chromatic dispersion.
For certain applications, the reduction of the bending losses is essential, especially when the fiber is intended to be stapled or coiled in a miniaturized optical box.
Hole-assisted fiber technology makes it possible to achieve excellent performances with respect to bending losses, but this technology is complex and expensive to implement and cannot be used for fibers intended for low-cost FTTH systems.
A need therefore exists for an optical fiber having a resistance to bending losses that is better (e.g., an order of ten times better) than the limits imposed by the G.657B standard. The fiber meeting this criterion should also remain compatible with the G.652 standard in terms of transmission profile and, in particular, mode field diameter. This appreciable improvement of bending losses may be achieved to the detriment of a higher cut-off wavelength, provided that (i) the directly higher order LP11 mode is sufficiently attenuated, and (ii) the length of fiber required for the attenuation of the LP11 mode to reach 19.3 dB at a wavelength of 1260 nanometers is less than 90 meters.